Two layers of mesh to create required Ast

[RicEll]

Hi!
I have a simple question... Are you allowed to lie a layer of SL92 (or any other mesh size for that matter) on top of another SL92 to double Ast? I have a simple slab and the installers have asked instead of tieing N12@200 EW together can he combine 2x SL92?

Thanks
Richard

 

[bugbus]

No issues with it that I can see, other than going from N to L class, which has its own limitations

 

[BridgeSmith]

As long as they're aligned, so that clear space for concrete to flow through is maintained...

 

[hokie66]

You can lose a bit of effective 'd' with two layers of mesh.

 

[rapt]

You lose significant effective depth and about 20% capacity due to decreased phi factor is it is a suspended slab!

 

[MotorCity]

If it has to be developed, I assumed you would have to treat it like bundled bars but you would need to check your code to see if that approach is permitted.

 

[rowingengineer]

I don't like this solution agree with the d and 20% but even if this stacks up, the over lapping isnt great especially at the 4 corner, you do need to spend the time to detail appropriately.

 

[Greenalleycat]

I don't like the solution personally.
Doubling up mesh leads to a lot of steel and less concrete
Round mesh has less effective bond than deformed bars so slip could cause issues - I'm interpreting your "simple slab" to be a simply-supported suspended slab, so cracking on the soffit may matter

For a slab-on-grade solution I would likely go for this, but not for suspended
Presumably the contractor priced on the bar solution and won't be offering a credit back, so make them do what they priced

 

[haynewp]

Where is the 20% reduction requirement located?

 

[Retrograde]

Where is the 20% reduction requirement located?

Check Table 2.2.2 in AS3600.

phi for N class reo is generally 0.85 whereas for L class reo it is 0.65

 

[Tomfh]

Yes you can stack mesh, but you need more mesh area compared to bars due to the penalty factors mentioned above.

You also need to align the mesh bar orientations to prevent the layers building up too quickly.

 

[haynewp]

Thanks. I don’t think we have a similar phi reduction for this case in ACI.

 

[rapt]

Haynewp,

Our mesh is low ductility,nominally 1.5% strain at failure.
At normal minimum steel levels, strain is about 4%.
Such a low strain at failure can lead to sudden collapse.

That is the reason for the reduced Phi.